Alright, so this is gonna be my personal, subjective opinion comparing the 3 mentioned headphones in a rather superficial way (i think there are more in-depth comparisons between these as they are all very popular headphones).
I first had the Sundara, which was one of my first hifi – headphones in general. Of course I was completely blown away by its sound quality (obviously, since I was coming from regular consumer IEMs and cheaper over-ears.).
Half a year later I bought the 6XX and found it to be a good addition to the Sundara, because of its gentle, non-fatiguing sound. Good for a very relaxed listen. But there were a few things “missing” with these cans for me..
About a year later I tested the R70x in a music-store and was instantly pleased by its sound; I bought it with a 30 day return window in mind. In the next days I decided to keep them, as they felt like a direct upgrade to the 6XX/650 in a few crucial ways.
For now I’m keeping all three, but will eventually sell the 6XX.
A bit more about the particular headphones and for what I mainly use them:
SUNDARA
I love these for everything with vocals and acoustic, natural instruments. It beats the other 2 in terms of timbre (naturality of sound - when i close my eyes it sometimes sounds as if I’m listening to the actual singer/instrument in front of me). It also has the best “detail-retrieval”, so it’s also the one I choose for more critical listening, especially in the treble area. Further the soundstage and imaging is impressive.
It does lack a bit of.. “organicness” <- yes, weird word. It feels more like a scalpel severing the music into many details and at times forgets to put the pieces together to create a coherence (which the other 2 do better). It is also heavier than the others with the cups floating around a little (but that's only an issue when you move around).
The second big use-case for me is watching movies/shows because of its great timbre and neutral-ish sound (never sounding boomy, which the other 2 occasionally do).
While the reduced bass is great for naturality it doesn’t work well for me with most of my electronic /synthetic music.
6XX
AS said before, very non-fatiguing (rolled of bass and rolled of treble). Very mid-centric with upper-bass/lower-mids hump. There is a lot of music which works well with this kind of tuning (especially older productions).
But the main caveat with these for me is its soundstage.. After having tried a few headphones over the time I just cant listen to the 6XX anymore because of its weird spatial presentation. When I apply HRTF convolution it does get a lot better (even listenable again), but it also alters the sound a fair bit. Everytime I put on these cans, after a few minutes I think.. nah, why not try the R70x instead.
R70x
So the R70x is the perfect allrounder for me in terms of frequency response and presentation. Overall it really feels to me like a cross between the Sundara and the 6XX taking the good properties (except for the planar magnetic treble-sparkle from the Sundaras).
As I mostly listen to electronic and trip-hop music, R70x are very satisfying for a range of different bands and artists. For really deep subbass-driven techno stuff (which probably makes up 60-70% of my listening time) I have 2 other cans. But the R70x has great bass-extension (for an open-back) and good treble-quality. The spatiality is more focused on depth instead of too much width like the 6XX does. Imaging is not far away from the Sundara's.
Another key factor is the comfort of these being feather-light and having a balanced clampforce (they almost disappear on my head).
PS: I’m mostly listening on my Fiio BTR5 Balanced output, but switch to SMSL SU/H6 stack when getting serious (all three 3 headphones have very similar power-demand).
So there you have it.. hope it helps you a bit on your “side-grading” adventure
Enjoy the music!